You can try matching the output impedance by sqeezing the box harder. How
many ohms is your hand + arm + torso + legs to ground?


For helicals of other dimensions download program HELICAL3 from website
below.

What do I use for a ground plane?

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YOU plus the box constitute the groundplane. Otherwise, for example, with a
set of 4 1/4-wave radials you won't be handheld.
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Reg